The foregoing description of the principal events in the development of the embryo will be sufficient for our purpose here. Further details on the subject would necessitate a considerable knowledge of physiology and anatomy, and those readers who desire to study the details of the subject further may do so in any of the various works referred to in the bibliography appended to this book. CHAPTER VII THE BEGINNINGS OF THINGS We may next turn our attention to the developing embryo at a very early stage, and note from which parts of its growing cells the different structures are ultimately developed, remembering all the while that all the subsequent division into specialised tissues is the result of the inherent possibilities in one single fertilised germ-cell. It will be remembered that, as the result of the subdivision of the fertilised germ-cell, we had the formation of three distinct layers of cells. These layers we saw were termed the germinal layers, and were named respectively the "ectoderm," the "entoderm," and the "mesoderm"--the last appearing between the two former. It is from these three germinal layers that all the subsequent structures of the body take their origin, and although we cannot attempt to follow out in detail the growth of all these special tissues, it will, nevertheless, be of interest to note, in the briefest possible way, from which portion of the embryo they subsequently arise. Some of these we may afterwards note in detail. The total result may be summarised by simply giving a list of the various tissues, and the corresponding embryonic layer from which they come. Thus:-- A. From the ectoderm arise the following structures:-- Epidermis or skin. The hair. Various glands. The lens of the eye. The whole nervous system. The nerve parts of the sense organs. The membrane of the mouth. The enamel part of teeth. The membrane of the nose. The lower part of the bowel. B. From the mesoderm arise the following structures:-- The connective tissues of the body. The bones. The teeth, except the enamel. All the muscles. All the blood-vessels of the circulation. All the vessels which carry lymph. The membranes of the heart, lungs, and bowels. The kidneys and their ducts. The reproductive organs. The blood itself. The fat and the marrow. C. From the third layer of the embryo, the entoderm, arises:-- The lining of the alimentary tract. The lining of the larynx. The lining of the trachea and lungs. The cells of the liver, the pancreas, the thyroid, and thymus. The structure termed the notochord.